In power transmission networks alternating current (AC) power is typically converted to direct current (DC) power for transmission via overhead lines and/or under-sea cables. This conversion removes the need to compensate for the AC capacitive load effects imposed by the transmission line or cable, and thereby reduces the cost per kilometer of the lines and/or cables. Conversion from AC to DC thus becomes cost-effective when power needs to be transmitted over a long distance.
The conversion between AC power and DC power is also utilized in power transmission networks where it is necessary to interconnect AC electrical networks operating at different frequencies. In any such power transmission network, converters are required at each interface between AC and DC power to effect the required conversion, and one such form of converter is a voltage source converter.
A typical voltage source converter is shown schematically in FIG. 1. The voltage source converter 10 includes first and second DC terminals 12, 14 between which extends a converter limb 16. Other voltage source converters may include more than one converter limb 16 and, in particular, may include three converter limbs each of which corresponds to a given phase of a three-phase electrical power system.
The converter limb 16 includes first and second limb portions 18, 20 which are separated by an alternating current (AC) terminal 22.
In use the first and second DC terminals 12, 14 are respectively connected to positive and negative terminals of a DC network, and the AC terminal 22 is connected to an AC network.
Each limb portion 18, 20 includes a chain-link converter 24 which extends between the AC terminal 22 and a corresponding one of the first or the second DC terminal 12, 14. Each chain-link converter 24 includes a plurality of series connected chain-link sub-modules 26.
Each chain-link sub-module 26 includes at least one switching element (not shown) which is connected in parallel with an energy storage device in the form of, e.g. a capacitor. The or each switching element may include a semiconductor device in the form of, e.g. an Insulated Gate Bipolar Transistor (IGBT), which is connected in anti-parallel with a diode although it is possible to use other semiconductor devices.
Switching of the or each switching element selectively directs current through the capacitor or causes current to bypass the capacitor such that each sub-module 26 is selectively able to provide a voltage. In this manner it is possible to build up a combined voltage across each chain-link converter 24, via the insertion of the capacitors of multiple chain-link sub-modules 26 (with each sub-module 26 providing its own voltage), which is higher than the voltage available from each individual sub-module 26.
The chain-link sub-modules 26 work together to permit the chain-link converter 24 to provide a stepped variable voltage source. This permits the generation of a voltage waveform across each chain-link converter 24 using a step-wise approximation. Operation of each chain-link converter 24 in this manner can be used to generate an AC voltage waveform at the AC terminal 22, and thereby enable the voltage source converter 10 to provide the aforementioned power transfer functionality between the AC and DC networks.